The objective of this research project is to demonstrate the presence of presynaptic cholinergic muscarinic receptors in the mammalian CNS. Neurotransmitter receptors in the CNS have traditionally thought to be localized postsynaptically; on dendrites or soma on neurons receiving synaptic inputs from nerve ending using a given neurotransmitter. Recent evidence suggests that presynaptic neurotransmitter receptors exist and may function physiologically to modulate transmitter release. In brain slices from the cerebral cortex atropine, an cholinergic antagonist, has been shown to enhance acetylcholine release while oxotremorine, a cholinergic agonist, reduces acetylcholine release. These and other pharmacological observations suggest that neurotransmitters, after release and interaction with the postsynaptic receptor, might then act upon the receptor site on the presynaptic neuron to prevent further transmitter release. An aim of this research is to determine the effectiveness of 3-quinuclidinyl benzilate (QNB), a potent muscarinic antagonist, in enhancing the release of (H3)ACh from hippocampal and interpeduncular brain slices. These are known cholinergic sites in the CNS. The existence of the septal-hippocampal and the habenulo- interpeduncular cholinergic tracts enable one to examine whether muscarinic cholinergic receptors are localized on presynaptic afferent neurons. If afferent neurons projecting to the hippocampus contain presynaptic receptors then destruction of the medial septal nucleus by electrolytic lesion should alter the receptor sites on the neurons. We will evaluate this by the use of the recently developed (H3)QNB binding assay. We will evaluate the kinetics of receptor binding since lesions may have altered the conformation of the presynaptic sites. We will also evaluate the feasibility of light autoradiographic localization of QNB binding sites on afferent neurons. The eventual direct visualization of the presynaptic cholinergic muscarinic receptors by light autoradiographic techniques should be of major interest.